Method of preparing a coating suspension



Sept. 18, 1945.

\ E. A. THURBER ETAL METHOD OF PREPARING A COATING SUSPENSION Filed Oct.22, 1941 FIG.

I 5. A. THURBE/P WVENTORS L. A. WOOTE/V y Wm $14M ATTORNEY PatentedSept. 18, 1945 METHOD OF PREPARING A oon'rmc SUSPENSION Elmer A.Thurber, Brooklyn,

Wooten, Maplewood,

Telephone Laborator York, N. Y., a co poration N. Y., and Leland A.

N. J., assignors to Bell ies,

Incorporated, New of New York Application October 22, 1941, Serial No.416,088

1 Claim.

This invention relates to coating suspensions and methods of preparationand more particularly to colloidal suspensions for coating electrodes ofelectron discharge devices.

In the manufacture of oxide coated emitters for electron dischargedevices and more particularly low current emitters of the filamentarytype for miniature devices employed in easily portable and inconspicuousapparatus, such as hearing aids or audiphones and personal radio sets ofthe handbag type, considerable difilculty has been experienced inobtaining emission efliciency and operating life comparable withconventional types of emitters in larger devices. In the miniaturedevices, the filament is of small diameter of the order of .001 inch to.0004 inch and due to such small size the usual coating procedures, suchas spraying or dipping are not practical from a commercial standpoint,and a more involved deposition process is required in which the coatingis applied or deposited by electrophoresis. In this process colloidalparticles of the carbonates are precipitated from a suspension and aredeposited on the fine filament after which the coating is decomposed tothe active state.

It has been found that the physical character of the coating or emittingsurface has an important bearing on emission, uniformity of surface,freedom from bombardment at higher anode potentials, arc-overs, andsputtering and that control of the physical character of the surfacepermits closer grid-cathode spacings to be attained. Furthermore, thepurity of the suspending medium, and the particle size and stability ofthe colloidal suspension have been found to be important factors in thesuccessful deposition of the coating by machine methods in manufacturingprocesses.

The principal object of this invention is to increase the percentage ofsmall particles in the emission layer applied to filamentary cathodes ofelectron discharge devices.

Another object is to maintain the purity of the suspension at thehighest degree to increase the electrophoretic efiiciency thuspermitting the coating to be deposited at low voltages and to assurestability of the particles in the coating mixture.

A further object of the invention is to overcome interfacial tension inthe coating process in order to effect adherence of the particles to thefine filamentary material.

Still another object of the invention is to increase the intensity ofemission of the coated cathode made with suspensions produced inaccordance with this invention.

Further objects are to maintain the electrophoretic properties of thesuspension particles at a premium value to increase the rate ofprecipitation in the coating process, to reproduce the optimum resultwith successive mixtures, and to minimize shrinkage in the coatingduring the decomposition and activating processes of the emittingsurface.

In accordance with a broad aspect of the invention, the suspension orcoating mixture is produced by mechanically disintegrating insulatingcompounds, such as alkaline earth compounds, in a. dispersing medium,such as a purified polyhydric alcohol vehicle having a viscosity equalto that of ethylene glycol under conditions which will result inproducing suspensions having high electrophoretic activity and in whichapproximately 50 per cent of the particles will not be greater than 0.5micron diameter.

As a specific example of the invention, mixed crystals of barium andstrontium salts, such as hydroxides or carbonates, are suspended in aviscous liquid, such as a mixture of glycerine and alcohol orvacuum-distilled ethylene glycol alone, in the ratio of 1 part salts to3 parts liquid and the mixture ground in a ball-mill with flint pebbles,the ratio of mixture to pebbles being 1 to 1 by volume and the agitationspeed being approximately 150 revolutions per minute for a period of to200 hours. After this treatment the stock mixture or concentrate isdiluted with anhydrous alcohol to a concentration of approximately 1.5grams per 100 cubic centimeters although the concentrate may be variedthrough a range of 1 to 5 grams per 100 cubic centimeters. The dilutecolloidal suspension forms an electrolyte which is placed in acataphoretic cell having an anode through which the filament to becoated is to be passed and a low potential is applied to thefilament-while passing through the cell for a definite travel time andtemperature to accurately control the thickness of coating applied tothe filament, the

particles being precipitated from the suspension in the cell to thefilament by cataphoresis under conditions which insure the highestdeposition emciency and uniformity of coating. This continuous coatingor machine method whereby quantity production of coated filaments. isachieved is possible in a great measure to the high ratio of colloidalsize particles in the coating mixture and the stability, lowconductance, cataphoretic properties of the particles in the suspension.

The method of preparing electrophoretically active suspensions bymechanically dispersing the alkaline earth crystals in the vehicle is tobe distinguished from the chemical method in which the compounds areprecipitated by a chemical reaction in essentially the medium or vehiclefrom which the particles are later to be deposited. The mechanicalmethod is more generally applicable and more easily controlled andtherefore more readily adapted to mass manufacturing processes.

The invention and its various features and ad-' vantages are moreexplicitly set forth in the following detailed description and will bemore clearly understood when considered with the accompanying drawingwhich shows in Fig. 1 a continuous coating procedure involving thecoating suspension of this invention and Fig. 2 is an enlarged detailview of the cataphoretic cell employed in the process of Fig. 1 andshown partly in cross section with the associated components for theprogressive movement of the filamentary conductor through the cell.

In the electrophoretic coating processes heretofore practiced,particularly on small diameter filaments, chemical and mechanicalcolloidal suspensions of alkaline earth salts have been employed withvarying success in small batches, but when attempts were made toincrease production of filaments to a large magnitude, various physicaldifiiculties ensued such as flocculation, rapid settling of carbonatesin suspension'and excessive shrinkage on drying. Also the weight-gain ofthe coating was difficult to control, the conductance was increased bycontamination of the mixture with impurities, and discontinuities ofdeposition of coating occurred which resulted in bare spots or thinspots. Other difilculties encountered in the development of a continuousprocess were non-uniformity of coating surface, slow rate of deposition,and unreproducible results; in some cases no deposition was obtainedprobably owing to high interfacial tension between the filament and thesuspending medium.

In accordance with this invention, these difliculties and disadvantagesare materially reduced or completely eliminated and the colloidalsuspensions or coating mixtures are highly stable, have highcataphoretic activity, increased intensity-of emission and overcomeinterfacial tension frictional forces of small dimension filamentspassing through the suspension fluid in the cell at fairly highdeposition speeds suitable for machine coating technique.

Mechanical disintegration offers the most practical solution to thereduction of particle size in a colloidal suspension fluid but seriousdifficulties are usually encountered by the introduction of impuritieswhich detract from the efficiency expected or the percentage ofcolloidal particles in thesuspension are not materially increased due toslipping and cushioning effects during disintegration caused by thecharacter of the dispersing fluid and the disintegrating media.

In order to obtain the required emciency in mechanical disintegration,the dispersing fluid should have a definite viscosity to preventslipping and cushioning effects. Furthermore, it should have the highestpossible purity, particularly with respect to the silica and alkalicontent being not greater than 0.10 per cent and preferably as low as0.025 per cent, since a higher impurity content reduces the cataphoreticproperties of the suspension and causes objectionable difficulties inemission of the finished coating on the filament. It has been found thatthe dispersing fiuid should have an optimum viscosity to obtain the bestresults. Examples are the polyhydric alcohols of which glycerine andethylene glycol are representative members. When glycerine is used it isdesirable to prepare a 50-50 per cent solution of glycerine and alcoholby weight to obtain the desired fluidity and when the glycol is employedit is preferable to distill under vacum to remove impurities.

In order to efficiently apply emissive coatings to filamentary cathodecores of small diameter, of the order of 0.0004 inch, theelectrophoretic process is ideally suited to this problem in whichhighly dispersed particles suspended in a fluid acquire a surface chargewhich causes them to migrate to an oppositely charged electrodev whensubjected to an electric field. The usual electron emitting coatingmaterial, such as barium and strontium carbonates, are lyophobic andtherefore difficult to prepare in stable colloidal form. However,mechanical disintegration or ball-milling may be employed with highlysatisfactory results to reduce the crystalline particles to a suitablesize for colloidal suspension.

This is accomplished in accordance with this invention, by preparing amixture of grams of mixed barium and strontium carbonate crystals with225 grams of vacuum-distilled ethylene glycol in a 500 cubic centimeterbottle half filled with clean riddled flint pebbles. The mixture isground by rotation in the usual bottle roller for a period from to 200hours at a speed of approximately revolutions per minute. The vacuumdistillation of the glycol may be carried out by mixing one-half poundof freshly crushed barium oxide, which serves as a dehydrating agent,with one gallon of glycol. This is agitated for one hour and allowed tosettle until only slightly turbid. The mixture is poured into afractionating still, preferably of Pyrex glass, and the heater andvacuum pump started to distill the glycol. The pressure should be about1 centimeter or lower so that the boiling point does not exceed 60 C.The purity of the glycol under these conditions is very high as shown byspecific conductivity of 4= 10-' mhos.

The glycol may be replaced by a solution 0 50 per cent glycerine and 50per cent alcohol to achieve similar results but it is not necessary toresort to vacuum distillation since this material is usually of highpurity. Similarly, mixtures of glycol and glycerine may be used as thedispersing medium. However, for cataphoretic coatings of satisfactoryadherence it is. necessary to remove the glycerine after theball-milling operation by centrifuging and to resuspend the sedimentedcarbonates in absolute alcohol.

The mixed crystals of barium and strontium and strontium in constantratio. These crystals c nsist of a solid solution of barium andstrontium carbonates formed by coprecipitation. However, other forms ofthe alkaline earth. compounds maybe employed toobtain varyingproportions of the carbonate.

-The percentage of.fine. particles of barium and strontium carbonates inthe concentrated suspension increased. considerably under the trifugingfor 400 seconds at 1000 revolutions per minute with the centrifuge tubesfilled with mechanical disintegration methods according to Table No. 1

Weight percent less than- Mechanical preparations A 6 hrs in glycol 6 55B 48 hrs in glycolw 10 75 C 100 hrs in glycol 30 85 D 200 hrs in glycol35 85 E 100 hrs in glycol 45 100 F 2d} hrs in glycerine and alcohol 3090 The highest per cent of particles with diameters less than 0.5 micronin sample E was attained with odd size riddled flint pebbles whereas inthe other examples uniform size oval' pebbles were employed. Sample Ealso shows that no particles in the whole suspension were larger than2.0 microns.

The relative effectiveness of ball-milling the mixed carbonates invarious media for various milling periods is shown in Table 2 in whichtheweight per cents of the solid aggregate remaining in; suspensionafter centrifuging under standard conditions are shown. The suspensionswere centrifuged in an International clinical centrifuge at 500revolutions per minute for 400 seconds with theeylindrical centrifugetubes filled to a height of 3' cm. or? cm. as indicated by R-S=3 and RS=7, respectively.

Table No.2

Percent in suspension Mechanical preparations Egg RS=3 R- S =7 Alcohol48 7. 4 24. Glycerine-alcohol 33 g 2 32' Glycol-glycerine 3g 2g: v 3 8Glycerine 100 10. 5 27. 0 48 20. 0 35. 7 Glycol 100 44. 0 62. 8 200 53.l 74. 0 Glycol 100 66.0

suspension to a height indicated by RS.;

, Table No. sv

.-R--Sincm. '1 2 i 3' -51 '1 1oo ias 17.5 220- 31.7 aao 100 '1o.1 as toass 2.5a loo knv-mg-fn' nn 10.1 13.8 15. 0 19.2 17.9

Winweight.percent. 18.5" 31.0 37.0 trod 60.0 Dinmm.X10- 0.31 0.45 0.560.15 0.94

gill is the weight per cent of particles with diameters less than D, isthe slope of the 1p vs. R-S curve and W is calculated by the followingequation:

D (in microns) =1.82-Jlogio WEEK) This analysis of the experimental datato obtain the particle size distribution is subject 'to the assumptionthat the suspension is free from vibrations during centrifuging and thatthe error caused by the use of cylindrical instead of sector-shapedtubes is negligible.

The particular efllclency of the coating suspensions made in accordancewith this invention is realized when machine or continuous coating ispracticed for'the mass production of filaments. Experience has shownthat suspensions having a high proportion of large particles do not havea satisfactory life foruse on a continuous coating machine. Such "asuspension may be used for a short time until the available supply ofparticles below a critical 40 maximum size, approximately 0.5 micron. is

exhausted. The. remaining particles are then too large or heavy toobtain herence at low voltages.

On the other hand the mechanical suspen sions as above described containsuch a large percentage of the critical size particles, especially forfilaments of the order of 0.4 mil, that the adherence to the movingfilament is greatly improved and a high speed of deposition is secured.Furthermore, the thickness of the coating is more easily controlled,depending on the time, temperature and voltage conditions duringdeposition, and it is possible to control the diameter of the coatedfilament to 0,001 inch 1.00005 inch with case at a relatively highdeposition rate.

A typical coating procedure by machine method is shown in Fig. 1; inwhich a small diameter filament ill, for example, of tungsten,'

of the order of 0.4 mil, is unwound from a supply reel II and passesbetween slack adjusting rollers 12 to an electrophoretic cell l3, shownmore clearly in Fig. 2. This cell involves a deep glass beaker I4,filled with a mechanical dispersing suspension l5 which is made up of1.5 grams of the ball-mill stock concentrate in cubic centimeters ofanhydrous ethanol or other suitable alcoholic fluid suspension. Thefilament enters the cell l3 through a glass sleeve I6 immersed in thesuspension, the sleeve being curved at the lower end to insureconcentricity of the filament which is made the cathode of the cell,with a tubular metallic anode I! having a perforated wall to facilitatethe flow of the suspension in the cell. It may be desirable tosatisfactory: ad'- substitute a mesh sleeve for the perforated metaltube to increase deposition.

The colloidal carbonate particles are precipitated fromthe suspensioncataphoretically by the application of a voltage source 18 shunted by apotentiometer IS, the negative side 01 the source l8 being applied tothe moving filament by a conductor 20 connected to a low resistancetungsten contact 2| engaging the filament and the sliding contact on thepotentiometer being connected to the anode I! by a conductor 22 havingan ammeter 23 in circuit to register the current flowing in the cell.The filament is guided by rollers 24 and 25 adjacent to the cell toinsure proper alignment of the filament passing through the cell.However, on the emergence of the coated filament from the cell, it isdesirable to apply a binder coating thereto so that the depositedcoating is not rubbed oil. This is accomplished by the capillary binderapplicator 26 through which the filament passes before reaching theroller 25. This applicator contains a few drops of a binder material 21,such as nitrocellulose in amyl acetate. The adherence is furtherenhanced by passing the filament through a container 28 positionedintermediate 9. pair of guide rollers 29 and 30 in which a suflicientbinder layer of nitrocellulose in amyl acetate is applied to protect thecoating when wound on the take-up reel 3| and during subsequenthandling. The schematic arrangement of the machine elements, as shown,simplifies the description and except for the substitution of the celll3, the coating machine is a modification of the type disclosed inUnited States Patent 1,986,533, issued January 1, 1935, to V. L. Ronciet al.

The cataphoretic deposition process facilitates the precipitation of theparticles of carbonates from the colloidal suspension due to the surfacecharge of the particles and the applied electromotive force between theanode l1 and the cathode or filament ill in the cell 13 so that theparticles adhere to the surface of the filament. The thickness of thecoating applied is easily controlled by regulating the voltage and timesince the rate of deposition is directly proportional to the current inthe usual range of coating thickness for small diameter filaments,namely, to microns. The average thickness 12 microns is approximatelyequivalent to 4 milligrams per square centimeter. As a typical example,a fine tungsten filament of 0.00041 inch diameter may be coated withbarium and strontium carbonate particles to a uniform thickness of 0.001inch by applying a low potential of 10 volts between the filament andthe anode in the cell for 10 seconds with the filament traveling at therate of 3.5 centimeters per second. The deposition voltage may vary from7.5 volts to 15 volts and the rate and duration of coating will beproportional to the voltage, depending on the different diameters offilaments passing through the cell.

The increased rate of coating will be realized from a comparison withprior chemical suspensions in which the maximum coating rate for 0.4 milfilament was 0.5 centimeter per second so that the present suspensionincreases the coating rate eight-fold. Furthermore, a greater amount offilament may be coated in a given quantity of suspension in the cell dueto the high percentage of colloidal particles contained in thesuspension. The resulting coating is of greater purity and compactnessso that a smooth uniform layer of controllable thickness may be obtainedon the filament.

While the invention has been directed specifically to the coating offine tungsten filaments with a colloidal suspension of barium andstrontium carbonates of high colloidal particle content employing highpurity viscous carrier materials as the disintegrating and dispersingmedium, it is, of course, understood that other electron emissivecompounds or materials may be employed in producing the electrophoreticsuspensions in accordance with this invention. Furthermore, othernormally insulating c01- loidal suspensions may be produced inaccordance with the mechanical disintegration process of this inventionfor the cataphoretic machine coating of cathode heater wire with aluminaor similar insulating materials or the deposition of various films ongrid wires, to counteract primary or secondary emission, and coatings onanode parts, such as zirconium, to dissipate heat in the operation ofdischarge devices. Therefore, the invention is to be construed inaccordance with the manner of treatment and mode of procedure inpreparing the colloidal suspensions to attain the desiredcharacteristics for the deposition of colloid particles of the coatingsubstances on fine wires by electrophoresis.

What is claimed is:

The method of preparing aggregate suspensions having a high percentageof colloidal particles from carbonates of relatively large sizeparticles for deposition by electrophoresis on filamentary wire of smalldiameter, which comprises grinding crystalline barium and strontiumcarbonates for an extended period from to 200 hours in a dispersingmedium of vacuum distilled ethylene glycol with riddled flint pebbles ina ball-mill whereby the particles are reduced in diameter to 2.0 micronsor less and from 35 to 50 per cent of the aggregate has a particle sizeof 0.5 micron in diameter, or less.

ELMER A. THURBER. LELAND A. WOO'I'EN.

